In general, an image sensor is a device that converts optical images into electrical signals.
FIG. 1 is a circuit diagram illustrating a photodiode unit pixel of an image sensor according to a related art.
According to the related art, when light is incident into a photodiode (PD) 110 in the state where transistors (TRs) of all pixels are turned off, signal electrons are generated and accumulated in an integration state in the photodiode 110.
At this point, a selective TR (Sx) 160 is turned on, so that a predetermined pixel 100 is selected. After that, a reset TR (Rx) 140 is turned on to reset noises of a floating diffusion (FD) 130 through a VDD terminal. At this point, the voltage of the FD 130 is output as a first output voltage V1 through a drive TR (Dx) 150.
Then, a transfer TR (Tx) 120 is turned on, and signal electrons in a fully integration state in the photodiode 110 are transferred to the FD 130. At this point, a second output voltage V2 is output.
A final output signal is output as a value of V1-V2.
According to the related art, since the capacity of the photodiode 110 is limited, when 100% light is incident, all signal electrons corresponding to the 100% light are not stored, e.g., the photodiode 110 is saturated by about 85% signal electrons and the other 15% signal electrons overflow into the FD 130. Thus, the signal electrons overflowing into the FD 130 are all disappeared during the reset operation.
Thus, there exists a need in the art for an improved image sensor capable of increasing the dynamic range of the image sensor.